1. Field of the Invention
This invention relates generally to an electrical system in which a power circuit is opened in the event that arcing current in excess of a predetermined magnitude is detected, and more specifically, this invention relates to an electrical system in which power is removed from an appliance or an entire house when arcing current in excess of a predetermined magnitude occurs, such as that resulting from the failure of insulation about a power line in the appliance cord or between lines of a house supply circuit.
2. Description of the Prior Art
In many instances in which people interact with electrically powered equipment (referred to generally herein as appliances), there is a possibility of the operator of the appliance being exposed to potentially dangerous electrical shocks, as well as the production of electrical fires. Similarly, an entire house and the members of the household can be exposed to such risks if there are failures in the electrical supply circuitry of the house. Although circuit breakers and fuses have been used for many years to protect electrical appliances, operating personnel, houses, members of the household and the surrounding environment from very large electrical currents and voltages, it is only within the relatively recent past that greater attention has been directed to the protection of individuals and property from less catastrophic, but still potentially dangerous, electrical shocks and fire hazards.
One type of protective device that is coming into much more general usage is the ground fault circuit interrupter (GFCI). A GFCI is used to protect against an undesired grounding of a power line, such as by a person inadvertently being connected from the power line to ground at an outlet in the home. A common form of such a ground fault circuit interrupter includes a differential transformer with opposed primary windings, one primary winding being associated with a power line and the other being associated with a neutral return line. If a ground fault should occur on the load side of the GFCI, the magnetic flux of the two primary windings will not cancel, with the result that a flux flow is produced in the core of the differential transformer. This resulting flux flow is detected by a secondary winding on the differential transformer core, and the secondary winding then produces a trip signal for a circuit opening arrangement to open the power line.
Besides GFCIs, other personal protection devices include appliance leakage current interrupters (ALCIs), equipment leakage current interrupters (ELCIs) and immersion detection circuit interrupters (IDCIs). Underwriters Laboratories, Inc., in its Reference Standard UL943A, has jointly characterized GFCIs, ALCIs ELCIs and IDCIs as Leakage Current Protection Devices. Whenever the term GFCI is used herein, it is intended in the broader sense of a Leakage Current Protection Device, where applicable.
While such GFCI applications have protected many people from serious injury or death in the event of a ground fault or leakage current, there are other types of hazardous situations that may not be protected against by the basic GFCI circuit. Thus, for example, in U.S. Pat. No. 4,598,331, arrangements are shown for protecting against potentially hazardous situations such as an open neutral or ground lead, an excessive voltage between the neutral lead and the ground lead, and reversal of input connections between the power and neutral lines. However, there are concerns with respect to electrical power cords and equipment other than these situations that are potentially hazardous to a person using the equipment.
One such concern relates to the arcing between a power line and a neutral line, which could also include leakage currents above a certain magnitude. Such electrical arcing may not be a direct threat to an individual person, but it can result in combustion. The resulting fire is potentially hazardous to both health and property. A GFCI with protection against this type of arcing threat in an electrical power cord or cable having a metallic sheath or cover is disclosed in U.S. Pat. No. 4,931,894.
However, arcing is not a threat solely in the situation where a metal sheath is utilized in a cord or cable, but is an equal, if not greater problem, where the power line and the neutral line are not shielded by a metallic sheath. In U.S. Pat. No. 4,931,894 (col. 3, lines 1-10), reference is made generally to the use of a sensing lead, either separate from or in addition to the metallic sheath. In a three lead system the ground lead may be utilized as the sensing lead. In addition, arcing or leakage current protection of this type is desirable as a separate feature, as well as in conjunction with a GFCI.
One area of particular concern are the appliance cords utilized to convey power to an appliance. These cords may be exposed to frequent bending or the placement of heavy objects on them that can, particularly over time, break down the insulation surrounding the power line and the neutral line. As the insulation breaks down, leakage currents and arcing can occur between the power line and the neutral line. The Consumer Product Safety Commission contends that hundreds of fires are started each year by such faulty appliance cords.
Even if the appliance cord incorporates a GFCI, or is plugged into a GFCI, arcing on the load side will not unbalance the differential transformer and thus will not be detected by the GFCI. As these potential arcing conditions could be hazardous to health and safety, it is desirable to protect against them, as well as the ground fault or leakage currents that would be detected by a GFCI.
Since it is desirable to detect arcing currents below the combustion level, it means that it is necessary to detect relatively small currents. The development of an arc occurs in three phases: (1) leakage current; (2) tracking; and (3) actual arcing. It is preferable to open the circuit before arcing actually occurs, most preferably in the first phase of a leakage current, which is the goal of this invention.
Trying to detect small leakage currents at a distance could involve undesired nuisance tripping. Thus, it is desirable to have the protective arrangement in the appliance cord, rather than being associated with the electrical supply end of the circuitry.
In the case of a house electrical supply circuit, it would be desirable to have interruption of the circuit achieved at the point where electrical power is introduced into the building to minimize the chances of fire.